Production of polyhydroxyalkanoates (PHAs), a family of naturally occurring renewable and biodegradable plastics, in crops has the potential of providing a renewable source of polymers, chemical intermediates and bio-energy from one crop if plant residues remaining after polymer isolation are converted to liquid fuels and/or energy. PHAs can provide an additional revenue stream that would make bioenergy crops more economically viable.
PHAs are a natural component of numerous organisms in multiple ecosystems and accumulate in a wide range of bacteria as a granular storage material when the microbes are faced with an unfavorable growth environment, such as a limitation in an essential nutrient (Madison et al., Microbiol. Mol. Biol. Rev., 1999, 63, 21-53; Suriyamongkol et al., Biotechnol Adv, 2007, 25, 148-175). The monomer unit composition of these polymers is largely dictated by available carbon source as well as the native biochemical pathways present in the organism. Today PHAs are produced industrially from renewable resources in bacterial fermentations providing an alternative to plastics derived from fossil fuels. PHAs possess properties enabling their use in a variety of applications currently served by petroleum-based plastics and are capable of matching or exceeding the perfounance characteristics of fossil fuel derived plastics with a broad spectrum of properties that can be obtained by varying the monomer composition of homo- and co-polymers, or by manipulating properties such as molecular weight (Sudesh et al., Prog. Polym. Sci., 2000, 25, 1503-1555; Sudesh et al., CLEAN—Soil, Air, Water, 2008, 36, 433-442).
Industrial production of PHAs in crop plants would provide a low cost, renewable source of plastics. Production of PHAs in plants has been an as yet unsolved goal for plant scientists and has previously been demonstrated in a number of crops unsuitable for industrial production or in industrially useful crops at levels to low to be commercially attractive [for review, see (Suriyamongkol et al., Biotechnol Adv, 2007, 25, 148-175); (van Beilen et al., The Plant Journal, 2008, 54, 684-701) and references within] including maize (Poirier et al., 2002, Polyhydroxyalkanoate production in transgenic plants, in Biopolymers, Vol 3a, Steinbuchel, A. (ed), Wiley-VHC Verlag GmbH, pgs 401-435), sugarcane (Purnell et al., Plant Biotechnol. J., 2007, 5, 173-184), switchgrass (Somleva et al., Plant Biotechnol J, 2008, 6, 663-678), flax (Wrobel et al., J. Biotechnol., 2004, 107, 41-54; Wrobel-Kwiatkowsk et al., Biotechnol Prog, 2007, 23, 269-277), cotton (John et al., Proceedings of the National Academy of Sciences of the United States of America, 1996, 93, 12768-12773), alfalfa (Small et al., Crop Set., 2002, 42, 919-927), tobacco (Arai et al., Plant Biotechnol., 2001, 18, 289-293; Bohmert et al., Plant Physiol., 2002, 128, 1282-1290; Lossl et al., Plant Cell Reports, 2003, 21, 891-899; Lössl et al., Plant Cell Physiol, 2005, 46, 1462-1471), potato (Bohmert et al., Plant Physiol., 2002, 128, 1282-1290), and oilseed rape (Valentin et al., Int. J. Biol. Macromol., 1999, 25, 303-306; Slater et al., Nat. Biotechnol., 1999, 17, 1011-1016.). Most of the efforts to produce PHAs in plants have focused on production of the homopolymer P3HB or the copolymer poly-3-hydroxybutyrate-co-3-hydroxyvalerate (P3HBV). While there have been some efforts to produce medium chain length PHAs in plants, these studies have yielded barely detectable levels of polymer (Romano et al., Planta, 2005, 220, 455-464; Mittendorf et al., Proceedings of the National Academy of Sciences of the United States of America, 1998, 95, 13397-13402; Poirier et al., Plant Physiol., 1999, 121, 1359-1366; Matsumoto, Journal of Polymers and the Environment, 2006, 14, 369-374; Wang et al., Chinese Science Bulletin, 2005, 50, 1113-1120).
To date, the highest levels of polymer have been obtained when the homopolymer poly-3-hydroxybutyrate (P3HB or PHB) is produced in plastids (Suriyamongkol et al., Biotechnol Adv, 2007, 25, 148-175; van Beilen et al., The Plant Journal, 2008, 54, 684-701; Bohmert et al., Molecular Biology and Biotechnology of Plant Organelles, 2004, 559-585). This is likely due to the high flux of acetyl-CoA, the precursor for PHB in these organelles during fatty acid biosynthesis (Bohmert et al., Molecular Biology and Biotechnology of Plant Organelles, 2004, 559-585). Expression of three genes encoding β-ketothiolase, acetoacetyl CoA reductase, and PHA synthase, allows the conversion of acetyl-CoA within the plastid to PHB. Previous work has reported producing levels of PHB in Brassica napus up to a maximum of 7.7% of seed weight, a level too low for commercial production
Therefore, it is an object of the invention to provide methods and compositions for producing transgenic oilseeds having commercially viable levels of polyhydroxyalkanoates in the seed, for example greater than 7%, 10%, 15%, or 19% polyhydroxyalkanoate or more of the total dry seed weight and capable of germinating.